Ipsilateral lymph nodes were obtained from patients undergoing radical mastectomy as well as simultaneous peripheral blood samples. In 30 patients leukocyte migration was carried out against a homologous breast cancer antigen obtained from a patient with an intraductile carcinoma of the breast. The same antigen was used in all pts. studied. A control group of 15 patients who had open heart surgery had blood samples and inguinal lymph nodes removed prior to heart surgery. These lymphocytes were tested against the homologous breast cancer antigen. Our results demonstrated that in the control group the peripheral blood lymphocyte population had a migration inhibition index (M.I.) of - 2% (S.E. plus or minus 3%) The lymph node lymphocyte population in the control group demonstrated a M.I. index of minus 10% (S.E. plus or minus 2%) demonstrating no significant response to the homologous cancer antigen. In the breast cancer group the peripheral blood lymphocytes had a M. I. index of 7%(S.E. plus or minus 3%). This was not different from our control group. However, the lymphocytes of the ipsilateral lymph nodes in the breast cancer group had a M.I. index of 21%(S.E. plus or minus 3%). The difference of a M.I. index of 21% in the ipsilateral lymph node lymphocytes and a M.I. index of 7% in the peripheral blood lymphocyte populations in the 30 patients undergoing radical mastectomy indicate that cellular reactivity was more marked in the lymph node lymphocytes than the peripheral blood lymphocytes. A M.I. index of greater than 20% is considered to be indicative of sensitization. In the breast cancer group there was 23/30 patients who had a M.I. index greater than 20% as compared to only 10/30 patients had a M.I. index greater than 20% when the peripheral blood lymphocyte population was analyzed. No patients in the control group had a M.I. index greater than 20%. This observation noted above implies that cellular reactivity is different when the two populations are compared suggesting immune reactivity in the ipsilateral lymph nodes.